Cold Squirts

Antarctic scallops have lighter shells, less muscle mass, and more resilient hinge rubber than their tropical cousins.

Squids and octopuses are well known for their jet-propelled locomotion, scooting along by squirting water out of their mantles. But bivalves? Not many people have seen the ungainly, clapping flight of the scallop, but its motion is likewise jet-propelled.

The scallop is one of only a few bivalve mollusks—invertebrates with a two-part shell—that can truly swim. When threatened, the scallop claps the two halves of its shell together, and thus expels a jet of water that propels it to safety. By repeatedly slamming the shell, the scallop manages to wobble unsteadily through the water.

Simple enough, right? Yet it probably won't surprise regular readers of this column that basic research on the locomotion of scallops has implications for scientific investigations of seemingly unrelated matters. It turns out that studying the swimming of cold-water scallops can guide applied research on manipulating polymers at various temperatures.

Like clams and mussels, scallops have two half shells, or “valves,” attached to each other by a strong hinge. A large (and tasty) muscle, the adductor, is attached to the center of each valve, and when the muscle contracts, the shell closes to protect the animal's soft parts. The muscle can exert force only to close the shell; to open, the shell relies entirely on a little rubbery pad of protein just inside the hinge. The rubbery pad gets squashed when the shell closes, but as the closing muscle relaxes, the pad rebounds and pushes the shell back open. That's why when you're shopping for live bivalves for dinner, you want the closed ones: they are manifestly alive because they're still holding their shells tightly shut.

The jetting mechanism in a scallop works like a somewhat inefficient two-cycle engine. When the adductor muscle closes the shell, water squirts out; when the adductor relaxes, the rubbery pad pops the shell back open, allowing water back inside and replenishing the jet [see illustration below]. The cycles repeat until the scallop is out of predator range or closer to a better food supply. Unfortunately, the jet-power phase is delivered for only a short part of the cycle. Scallops, however, have adapted to make the most of what power and thrust they can produce.

One of their tricks is to lighten the load by having thin shells, whose weakness is offset by corrugations. Another adaptation—the key, in fact, to their culinary charm—is that large, tasty adductor muscle, physiologically suited to the powerful cycles of contraction and relaxation in jetting. Finally, that little rubbery pad is made of a natural elastic called abductin, which does an excellent job of returning the energy put into it by shell closure.

As inefficient as jetting is for all scallops, the cold-water species face even tougher challenges. For one thing, the power output of muscles decreases in the cold. For another, cold water is more viscous, and offers more resistance. And finally, in the Antarctic, where the water temperature is only twenty-eight degrees Fahrenheit, the rubbery abductin should become less elastic. Those factors explain why the Antarctic scallop, Adamussium colbecki, is just barely able to sustain level motion.

Yet despite the cold, A. colbecki manages to swim. Mark W. Denny and Luke P. Miller, biomechanists at Stanford's Hopkins Marine Station in Pacific Grove, California, traveled all the way south to McMurdo Sound to figure out how. Their initial findings were not unexpected: in A. colbecki the shell contributes less to the animal's total weight than it does in tropical scallop species, giving its adductor muscle less shell to swing shut with each jet cycle.

Denny and Miller's next set of measurements, however, is harder to understand. Instead of an extra-large muscle to compensate for the cold, they found that A. colbecki has a closing adductor half as big as the adductor in a warm-water bivalve of similar size. Although that, too, saves weight, the shift in proportions implies that closing the shell takes less force but more time—not to mention that it takes more cold-water scallops to make a satisfying entrée. In fact, the combination of low shell mass and low muscle mass translates into a severe handicap for the scallop—a ratio of jetting power to animal mass that is only 20 percent that of the warm-water scallop's. Those numbers explain why cold-water scallops are just barely able to jet.

Of course, Denny and Miller were on the lookout for some evolutionary advantage to make up for the skimpy musculature. What they found was something new about the properties of polymers.

In severe cold, the abductin in the scallop's hinge should become less able to store energy. After all, as many readers may recall, the catastrophic explosion of the space shuttle Challenger in 1986 was caused by cold weather, which made the booster-rocket O-rings so hard and brittle that they allowed hot gases to escape. When Denny and Miller checked the effect of changing temperature on the Antarctic scallop's abductin, they did indeed find a decrease in bounce with a drop in temperature, but it was a smaller decrease than occurs in temperate-zone mollusks. Natural selection has thus fine-tuned the response of abductin to temperature.

The difference is small potatoes for the scallop; the energy returned by Antarctic abductin is only a small fraction of the total needed to jet. But a rubber that retains its bounce in the cold would make materials scientists take notice.

The scallop won't readily give up its secret, however. The composition of warm- and cold-water abductins is basically the same; the differences must lie in the arrangement of the protein polymers that store and release energy. Identifying those minute differences will further confirm the rule of thumb that blue-sky (or in this case blue-water) research has unanticipated implications far outside the shell of the original work.

Antarctic scallop escapes capture by jet propulsion. The creature launches itself by closing the two halves of its shell with its adductor muscle. The closing action forces water out of the shell's interior and compresses the rubbery hinge tissue (green). As the hing tissue rebounds, the shell slowly opens, water re-enters the shell, and the muscle returns to its initial position, ready for another thrust. Blue represents the movement of water, and red represents the force of the adductor muscle.